Building Services Engineering 5th Edition Handbook

222 Surface-water drainage
Table 8.1 Ground impermeability factors.
Nature of surface
Impermeability factor
Road or pavement 0.90
Roof 0.95
Path 0.75
Parks or gardens 0.25
Woodland 0.20
The drain water flow rate Q is given by:
Q = area drained m 2 × rainfall intensity mm h
× impermeability factor
EXAMPLE 8.1
Footpaths, roadways and gardens on a commercial estate cover an area of 75 000 m 2 ,
of which 20% is garden and grassed areas. Estimate the surface-water drain flow load in
litres per second. How many 15 l/s surface-water drain gulleys are needed?
From Table 8.1, impermeability factors are 0.9 for the roads and paths and 0.25 for gardens
and grass. Therefore:
Average impermeability = 0.2 × 0.25 + 0.8 × 0.9
Hence,
= 0.77
Q = 75 000 m 2 × 50 mm h × 1h
3600 s × 1m
10 3 mm × 0.77 × 103 l
1m 3
= 802.1 l/s
Number of drain gulleys needed
= 802.1 l s × gulley s
15 l
= 54 gulleys
Roof drainage
A rainfall intensity of 75 mm/h occurs for about 5 min once in 4 years. An intensity of 150 mm/h
may occur for 3 min once in 50 years, and where overflow cannot be tolerated this value is used
for design. The flow load Q for a roof is calculated from:
Q = A r m 2 × 75 mm h × 1h
3600 s × 1m
10 3 mm × 103 l
1m 3
= A r m 2 × 0.021 l/s